An emulsion is widely used in various fields such as foods, agriculture chemicals, pharmaceutical products, cosmetics, synthetic resins, and paints, wherein a stabilizing technology of an emulsion is required. That is, a method for producing an emulsion having uniform particle size is required to keep an emulsion stably.
The effect of an emulsion having uniform particle size on its stabilization can be explained by Ostwald aging based on the following Thompson-Freundlich equation (Non-Patent Document 1). According to this, solubility of a dispersed particle near its interface into continuous phase is dependent on curvature of the interface, wherein the solubility becomes higher with a smaller particle size and becomes lower with a larger particle size. When emulsions having different particle sizes are present, the interfacial area of the emulsion decreases to stabilize the emulsion, resulting in concentration difference between particle's surface having large solubility and particle's surface having small solubility, thereby causing mass transfer that leads to downsizing of small particles further smaller and gradual growing of large particles further larger. In monodisperse emulsion having uniform size, there is no concentration difference in the particle interface, and thus the mass transfer can be neglected. In other words, production of an emulsion having uniform size is necessary to keep an emulsion stably, and is important also for realizing functions of emulsion effectively.
[Equation 1]RTln(Ca/Cp0i0)=2(γ/a)(M/ρ)  (Thompson-Freundlich equation)Ca is solubility at the interface with curvature a; Cp0i0 is solubility at horizontal interface; γ is surface tension; M is molecular weight of dispersed phase; and ρ is density of dispersed phase.
Various methods have been proposed for producing, for example, an O/W emulsion in which an oil phase (dispersed phase) is present as microparticles in a water phase (continuous phase) and a W/O emulsion in which a water phase (dispersed phase) is present as microparticles in an oil phase (continuous phase), wherein the variation coefficient (CV) shown by the following equation derived from an average particle diameter and a standard deviation in particle size distribution of emulsion particles is used as an indicator to express uniformity of obtained emulsion.
[Equation 2][standard deviation]÷[average particle diameter]×100=[variation coefficient](%)
When the variation coefficient is smaller, particle size distribution becomes narrower and monodisperse, making an emulsion higher uniformity. On the contrary, uniformity as an emulsion becomes lower as the variation coefficient becomes larger, so that there occasionally appears such a problem that emulsion particles having particle diameter outside the required size need to be removed by classification operation in industrial production.
Emulsion production methods known conventionally include a method using a mechanical shear force (Patent Document 1, Patent Document 2, and Patent Document 3), a method using a solid membrane (Patent Document 4), a method using a ultrasonic wave, and a method in which a dispersed phase is passed through a flow path treated by a interfacial-chemical treatment such as a microchannel (Patent Document 5).
In the case that mechanical shear force is applied by agitation, flow of the fluid in an agitated tank tends to be nonuniform independent of kinds of an agitator and an agitator blade. Accordingly, an excess energy is applied to a material to be processed and it becomes difficult to give energy evenly to every particle, thereby tending to result in wider particle size distribution of the obtained emulsion. The same is true for a high pressure homogenizer and an ultrasonic disperser; in these methods there also occasionally appear such problems as increase of energy requirement, increase of running cost, and complication of operational process in order to obtain fine emulsion particles. In addition, an ultrasonic disperser has a problem of difficulty in up-scaling to actual production.
In the case that a solid membrane or a general microchannel is used, relatively uniform particles can be obtained, but particle size of obtained emulsion is strongly affected by surface characteristics of construction material of the apparatus, wetting properties, and pore diameter, and thus a construction material and a pore diameter need to be changed according to the purpose and its use. In the case that a general microchannel is used, the pressure loss inversely and quadruplicately relates to a flow path as the diameter of flow path becomes narrower and narrower, and thus a pressure required for feeding a fluid becomes extraordinary so large that a pump to feed a fluid might not be available easily. On top of that, as to up-scaling, it has been done by “numbering-up”, that is a method in which numbers of microchannel itself are increased. However, in practical viewpoint, multiplying of numbers is limited to several tens of microchannels; this naturally leads the target to a product of high value. In addition, when a problem such as clogging occurs, there is a possibility that identification of the problematic site such as the site of malfunction be extremely difficult.
Non-Patent Document 1: “New Methods for Obtaining Highly Stabilized Emulsions”, ed. by Toshiyuki Suzuki, Gijutu Joho Kyokai (Technical Information Institute Co., Ltd.), May 2004
    Patent Document 1: JP-A 2000-218153    Patent Document 2: JP-A H06-142492    Patent Document 3: JP-A 2006-26457    Patent Document 4: JP-A H07-100347    Patent Document 5: JP-A 2007-54681
In view of the problems as described above, the present invention has an objective to provide a uniform emulsion that can be produced with a low energy and does not require classification operation.